Lithium ion secondary cells have been improved in performance in recent years for use as power sources for mobile devices. Lithium secondary cells are expected to be useful in the field of peak-shift power sources and motor vehicle assisting power sources from the viewpoint of the energy density of the secondary cell, and it is desired that these cells be further improved in service life and reliability. Lithium secondary cells presently available generally comprise a ceramic oxide serving as a positive electrode active substance for lithium ions to be inserted thereinto and released therefrom, metallic lithium or a lithium alloy, or a carbon material or silicon material for absorbing and desorbing lithium ions for use as a negative electrode, and an electrolytic solution containing a lithium salt as dissolved in an organic solvent.
Electrolytic solutions comprising an organic solvent and heretofore in use are inferior in electric conductivity to aqueous electrolytic solutions for use in lead batteries or the like and still remain to be improved in the internal resistance of cells for uses wherein a high output is required.
Salts melting at room temperature (so-called, room temperature molten salts) are liquids consisting only of ions, have characteristics of being nonvolatile and flame-retardant and have been expected to be applicable to electrolytic solutions for lithium secondary cells.
Typical examples of room temperature molten salts include 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI. BF4). However, the imidazolium salt decomposes at a nobler potential than lithium and is therefore difficult to apply to lithium secondary cells. Studies are under way in recent years on room temperature molten salts and remaining stable over a wider range of potentials.
For example, Nonpatent Literature 1 has revealed that salts comprising bis(trifluoromethanesulfonyl)imide anion and an aliphatic ammonium cation include compounds having a melting point not higher than room temperature and exhibiting improved electrochemical stability.
Patent Literature 1 further shows that aliphatic quaternary ammonium room temperature molten salts are applicable to lithium secondary cells. However, Patent Literature 1 merely discloses that lithium will deposit and dissolve on a nickel substrate in the aliphatic room temperature molten salt, but discloses nothing about any technique for using the salt for lithium secondary cells.
Nonpatent Literature 2 reveals that when graphite which is in wide use as the negative electrode material for lithium secondary cells is used as an electrode in a bath containing ammonium cations, the ammonium cations are decomposed or inserted in between graphite layers. In connection with these problems, Patent Literature 2 discloses a nonaqueous electrolytic secondary cell which is characterized by having a room temperature molten salt and having vinylene carbonate or like cyclic ester, and shows that the presence of the cyclic ester forms on the surface of the negative electrode material a protective film for inhibiting reductive decomposition of cations of the room temperature molten salt, consequently giving stabilized charge-discharge characteristics.
Patent Literature 3 and Patent Literature 4 disclose a nonaqueous electrolytic cell containing an aliphatic room temperature molten salt, and a nonqueous electrolytic lithium secondary cell having high safety and satisfactory charge-discharge characteristics.
Although the foregoing techniques suggest that stabilized charge-discharge behavior of high safety is available, the aliphatic room temperature molten salts and exemplified are all compounds having high viscosity, failing to afford electrolytic solutions which are fully satisfactory in electric conductivity even if the salts are used as admixed with an organic solvent of low viscosity. The use of the salts gives cells increased internal resistance especially over a low temperature range.    [patent literature 1] Japanese patent No. 2981545    [patent literature 2] JP 2003-373704 A    [patent literature 3] JP 2003-203674 A    [patent literature 4] JP 2003-288939 A    [nonpatent literature 1] Ionics,3,356(1997)    [nonpatent literature 2] Electroanalytical Chemistry and Interfacial Electrochem.,53, 329-333(1974)
An object of the present invention is to provide a nonaqueous electrolytic lithium secondary cell wherein an electrolytic solution of high electric conductivity is used and which has reduced internal resistance and stabilized charge-discharge characteristics.